I never played Super Mario Rpg, so I'm not quite sure what you're shooting for here. If you want to have button presses effect the strength/type/duration of your attacks in the dbs, it can be done. As a test project, I implemented a combo attack system, with the ability to accurately block attacks during the enemy's turn, using directional keys in Rm2k3's dbs.
The key is bouncing out of the battle events to common events to enable the "enter password" variable trapping for button presses. When the event finishes, you'll be back in the battle event, and you can set your conditionals and the attack types you want them to represent.
With a timer function and some go to label looping, you'll be able to kick the player out of the subsystem if he takes too long with the sequence.
The combination attacks are triggered by directional keys, as is the blocking during the enemy's turn. If this is sort of what you're looking for, I may be able to help get you started. It is fairly intensive though, and gets more so with each additional character that you have to track. The main hassle is working within the confines of battle events, so I recommend getting as many of your functions as possible shunted off to a common event, so you can just use the in battle pages for attack processing.